My house was built in 1953 in Nebraska. It is a ranch style home with a full basement. I am in the process of redoing my basement. Had an existing bathroom that i demoed out. It was not done very well. Plumbing was ran above the slab and tied into the main drain and vent pipe from up stairs by what my father in law said was a "saddle clamp". We are going to redo it right and tie everything in below the slab. Any ways, about 7 ft or so away from the main pipe there was a stub up. Was threaded pipe with a cap on it. Cut out the concrete thinking its was a clean out or something to tie a laundry drain into it only to find out the threaded pipe was only a short nipple that was placed (not screwed) into another belled pipe with fine threads. I'm not getting any sewage smell out of it. Can't tell where it goes either. Even shown a flash light down it and could not see the bottom. I would say it goes down at least 8 to 10 feet that I can see. About a three inch inside diameter pipe, the bell opening is 3 1/2 inches inside diameter with a ridge about 4 inches into it all the way around. The threads do not start right way. They start about 1/2 inch in. Just wondering what you all might think it is. Can I seal it up and forget about it? Or is it important to have access to?